Composite materials based on epoxy resins are used in a variety of applications and continue to have considerable importance because of their versatility. A specific example of such an application is in the production of electrical laminates used in printed circuit boards (printed wiring boards, PWB). A key requirement of this and many other applications is flame resistance. Accordingly, it has been customary in the preparation of epoxy-containing laminates to incorporate various additives to improve the flame retardancy of the resulting laminate. Many types of flame retardant substances have been used, however, the most common thus far used commercially have been halogen containing compounds, such as tetrabromobisphenol A. Typically, in order to reach the desired fire retardancy level (V-0 in the standard “Underwriters Laboratory” test method UL 94), levels of such bromine-containing flame retardant substances are required that provide a bromine content from 10 weight percent to 25 weight percent based on the total weight in the product.
Generally, halogen-containing fire retardant epoxy resins, such as those containing tetrabromobisphenol A, are considered to be safe and effective. However, there has been increasing interest in the industry to utilize flame-retarded epoxy systems that are not based on halogen chemistry. However, these replacement materials must still be able to meet the requirements of fire retardancy and to display the same advantages of mechanical properties, toughness, and solvent and moisture resistance that are offered by the halogenated materials currently used.
One alternative approach has been the use of phosphorus based fire retardants. See for example, EP 0 384 939 and U.S. Pat. Nos. 5,817,736; 5,759,690; 5,756,638, 5,648,171; 5,587,243; 5,576,357; 5,458,978; 5,376,453; and 5,036,135; all of which are incorporated herein by reference in their entirety. In all of these references, a formulation is formed from the reaction of a flame retardant derived from a phosphorus compound and an epoxy resin, which is then cured with an amino cross-linker such as dicyandiamide, sulfanilamide, or some other nitrogen element containing cross-linker to form the thermosetting polymer network.
Specific examples of commercially available phosphorus-based fire retardant additives include Antiblaze® 1045 (Albright and Wilson Ltd, United Kingdom) which is a phosphonic acid ester. Phosphoric acid esters have also been used as additives, such as, for example, PX-200 (Diahachi, Japan). Other commercially available reactive phosphorus containing compounds disclosed as being suitable for epoxy resins include Sanko HCA and Sanko HCA-HQ (Sanko Chemical Co., Ltd., Japan).
Alkyl and aryl substituted phosphonic acid esters are particularly compatible with epoxy resins. However, these phosphonic acid esters are often unsatisfactory as substitutes for halogenated flame retardants in epoxy resins for the production of electrical laminates. For example, these materials are known to be plasticizers and thus laminates formed therefrom tend to exhibit undesirably low glass transition temperatures (Tg). An additional drawback is that the use of phosphonic acid esters in amounts sufficient to provide the necessary flame retardancy increases the tendency of the resulting cured epoxy resin to absorb moisture. The moisture absorbency of cured laminate board is very significant, because laminates containing high levels of moisture tend to blister and fail, when subjected to the soldering operations typically employed in the manufacture of printed wiring boards.
Various other phosphorus based flame retardant materials are described in the literature, which are either too expensive or feature certain inferior properties. For example, EP 0 754 728 discloses a cyclic phosphonate as a flame retardant material, which is incorporated into an epoxy resin. However, the cyclic phosphonate must be present in large quantities, such as in excess of 18 weight percent, in order for the resin system to meet UL 94 V-0 rating. This loading for the phosphonate compound may lead to a depression of the Tg or higher moisture absorption. EP 1 116 774 utilizes a hydrogen phosphinate, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, in conjunction with triphenylphosphine oxide. However, the epoxy resin base requires the use of non-standard epoxy resins; namely a xylene-modified novolak resin and naphthylene aralkyl and biphenyl-modified epoxy resins.
Various other phosphorus compounds have also been used to prepare halogen-free flame retardant epoxy resins useful in the manufacture of composite materials. For example, the use of phosphorus-carbon bonded moieties, such as phosphine oxides, have been disclosed in WO 01/42253; U.S. Pat. No. 4,345,059; EP 1 116 774; JP2000186186 and JP 05057991B4; all of which are incorporated herein by reference in their entirety. Such phosphine oxides display benefits of improved resistance to moisture uptake when compared with other phosphorus compounds that contain P—O bonded moieties, as disclosed in WO 01/42253. However, a key disadvantage of these compositions is that they are costly to prepare, because they utilize unique raw materials. For example, JP2000186186 discloses the use of pure bis(p-hydroxyphenyl)phenyl-phosphine oxide, which requires the use of a pure dichlorophenyl phosphine in its production. Similarly, JP 05057991B4 discloses the production of tris-(m-gylcidyloxyphenyl)phosphine oxide by reacting the pure meta phenol with epichlorohydrin. In an analogous manner, the phosphine oxides utilized in WO 01/42253 require lithium reagents and cryogenic reaction conditions, thus requiring special equipment for its manufacture.
In U.S. Pat. No. 6,733,698 there is disclosed a mixture of hydroxyarylphosphine oxides comprising (a) a mono(hydroxyaryl)phosphine oxide, (b) a bis(hydroxyaryl)phosphine oxide, (c) a tris(hydroxyaryl)phosphine oxide, and, optionally (d) a tri-aryl, alkyl or aralkyl-substituted phosphine oxide. The mixture is produced by reacting a mixed Grignard reagent with phosphorus oxychloride and is said to be useful in the preparation of polyglycidyl ethers and as a flame retardant in epoxy resin compositions which can be processed into resin-impregnated composites.
U.S. Pat. No. 6,740,732 discloses phosphorus element-containing crosslinking agents for epoxy resin compositions based on isomeric mixtures of tris(2-hydroxyphenyl)phosphine oxides having the following general chemical structure:
wherein R may be independently a hydrogen or a C1-C10 alkyl group.
The present invention provides a novel composition comprising mixtures of ortho and para isomers of mono-, bis- and tris-(hydroxyphenyl)phosphine oxide compounds which is useful as a flame retardant in epoxy resin formulations. The composition is readily prepared from reaction mixtures of ortho and para halogenated phenolic ethers, e.g., a mixture of 2- and 4-bromoanisole, which mixture is conveniently and inexpensively obtained using a two step process whereby a phenolate salt is treated with an alkyl halide to produce phenol ether/haldie salt mixture followed by oxidation e.g., by addition of peroxide. The present composition is therefore significantly less expensive to produce than many of the phosphorous containing flame retardants suggested in the prior art, such as U.S. Pat. No. 6,733,698, yet endows polymer compositions with equal or better flame retardant and physical properties when the appropriate ortho to para ratios are selected.
It is possible to prepare compositions similar to those of the present invention comprising only the tris-(hydroxyphenyl)phosphine oxides of the present invention without the mono- and bis-(hydroxyphenyl)phosphine oxides found herein, however such mixtures do not always provide the full compliment of exceptional properties obtained when using the present compositions.